This invention relates to a rock drill air leg control system.
Conventional pneumatic rock drills have an air leg which is mounted pivotally to the drill proper. In use, the lower end of the air leg is spragged, i.e. engaged with the floor or footwall of a mine working where drilling is taking place. The air leg is supplied with compressed air from the same source as the rock drill proper and can be extended or retracted by appropriate manipulation of a twist-grip controlled valve. During extension, the spragged air leg supplies thrust to the rock drill which assists in the drilling operation When the air leg reaches the end of its design stroke, it has to be retracted and re-spragged closer to the working face before drilling can resume. This is achieved by manipulating the twist-grip controlled valve to regulate the supply of compressed air such that the air leg retracts.
A problem associated with the known system described above is that when the air leg is retracted using the twist-grip controlled valve the original thrust setting is lost. Accordingly, each time the drilling operation is restarted after re-spragging, time is wasted while the operator of the rock dill tries to get back to the previous thrust setting for optimal drilling.
In a more sophisticated arrangement exemplified by the applicants S250 rock drill a dual valve system is used. In this valve system there is a twist-grip controlled valve for regulating the trust setting and a separate, second valve for retracting the air leg when re-spragging is to be carried out. Since the twist-grip controlled valve need not be manipulated during re-spragging, the original thrust setting can be maintained.